Field of the Invention
The present invention relates to a gear shift control method of a vehicle, particularly, to a gear shift control method of a Dual Clutch Transmission (DCT) vehicle, which can promptly reflect acceleration intention of a driver in the DCT vehicle.
Background of the Related Art
A Dual Clutch Transmission (DCT) is configured such that two input shafts provided in two clutches to intermittently receive power and two output shafts respectively corresponding to the two input shafts alternately form gears according to a series of gear ratios, together with a separate shift mechanism. That is, power is transferred to one of the two input shafts, and the two input shafts and the two output shafts configure odd-numbered gears and even-numbered gears, respectively.
The most outstanding feature of the DCT is a fast gear shift without interruption of power. For example, if gears are sequentially shifted from a first gear level to a second gear level, i.e., a next gear level, a clutch connected to an input shaft currently combined with a first gear is released, and a clutch connected to another input shaft combined with a second gear is engaged immediately, and thus a fast gear shift without interruption of power can be implemented.
For the fast gear shift without interruption of power, which is the most outstanding feature of a DCT transmission, it is important to engage a target gear as soon as possible when gears are shifted. Particularly, in the case of a downshift reflecting acceleration intention of a driver (hereinafter, referred to as ‘kick down shift’), accurate gear shift control is required in the early stage of gear shift to promptly reflect the acceleration intention of the driver and overcome a feeling of discontinuity caused by the difference of gear ratio between gears when the gears are shifted.
To this end, a ‘release clutch engine control’ is performed in the early stage of the gear shift. Here, the ‘release clutch engine control’ is a control of increasing or decreasing torque of a clutch so that the engine speed, i.e., revolutions per minute (RPM)′, becomes equal to the synchronized speed of a target gear, and the ‘release clutch’ is a clutch for transferring power of the engine to a currently engaged gear, which is a release-side clutch whose connection is released when the gears are shifted.
If a kick down shift is made in the DCT, the power connection state between the engine and the current gear is released by decreasing torque of the release-side clutch, and contrarily, the engine and a target gear are connected by increasing torque of an apply clutch (hereinafter, referred to as a ‘connection-side clutch’), so that power of the engine is transferred to the target gear through the connection-side clutch and an input shaft connected to the connection-side clutch.
In the case of a generally known automatic transmission, torque transfer may be smoothly accomplished by applying the apply torque to the connection-side clutch as soon as the ‘release clutch engine control’ is completed. Contrarily, in the case of the DCT, a target gear should be necessarily engaged in order to apply the apply torque to the connection-side clutch.
If a target gear is not engaged although the ‘release clutch engine control’ is completed, the ‘release clutch engine control’ should be continued until a target gear is engaged, and the driver may feel that the torque is discontinued or the acceleration intention is not reflected. That is, the ‘release clutch engine control’ should be harmonized well with target gear engagement to implement the fast gear shift without a feeling of discontinuity.
For the fast engagement of a target gear, a method of controlling a hydraulic control solenoid valve by applying overcurrent may be considered. However, in this case, it is worried that severe noises and impacts to the vehicle may occur due to the supply of overcurrent, and since transfer of torque through the clutch is not accomplished smoothly due to the shaking of the input shaft because of the impacts when the impacts occur, gear shift quality is lowered greatly.
Particularly, in the case of a kick down shift in which the difference between the synchronized speed of a target gear and the actual revolving speed of a target gear input shaft is big, the speed of boosting the engine is faster although the gear is engaged as soon as possible, and thus unnecessary ‘release clutch engine control’ or ‘engine torque reduction control’ should be performed, and as a result, the driver may feel that the torque is not transferred properly regardless of the acceleration intention.
Furthermore, as the slip time between the release-side clutch and the engine is extended due to the ‘release clutch engine control’, it may be critical to the lifespan of the clutch due to abrasion or thermal deformation of the clutch. Therefore, in the prior art, revolutions per minute of the engine should be increased slowly as shown in FIG. 1, considering even the time of inserting a target gear when the kick down shift is made, and thus shift delay occurs, and a satisfying acceleration performance is not obtained.